This invention relates generally to the field of suspension systems, and in particular to suspension systems for wheeled vehicles, such as bicycles. More specifically, the invention provides suspension systems that are useful in connection with the rear wheel of a multi-wheel vehicle, such as with the rear wheel of a bicycle.
Suspension systems are widely used in a variety of vehicles. Many of these suspension systems are progressively becoming more elaborate. For example, one trend is to increase the sophistication of both rear wheel and front wheel bicycle suspension systems.
When designing a vehicle suspension system, a variety of parameters should be taken into account. Such parameters can include, for example, bump compliance, chassis roll control from braking or accelerating, unnecessary suspension movement caused by torque in the drive chain, frame weight, complexity, rigidity, and the like.
For example, when the rear wheel of a vehicle encounters a bump, a force that is directed upward and rearward on the wheel is produced. Therefore, a suspension design that permits the wheel to move upward and rearward is best at mitigating the bump impact. As another example, the chassis may tend to roll while braking or accelerating due to the rider's center of mass being located high on the vehicle. As such, when the brakes are applied, the rider's mass tends to continue forward and “roll” or “tip” over the chassis. The opposite occurs while accelerating. Hence, during acceleration, the wheels tend to drive out from under the rider, causing the chassis to tip backward. This tendency is exaggerated when climbing a steep hill. Hence, the force created when braking or accelerating tends to place more weight on one of the wheels and “lightens” the other wheel.
To compensate for such effects, a variety of suspension systems have been developed. Although many variations exist, most systems generally fall into three main categories. These categories may be based on the method of wheel articulation. The variations within each category usually focus on how a shock absorber is mounted and activated. Some systems have a simple strut-type mounting, while others employ complex linkages to gain a more progressive shock movement, or locate the shock to a more convenient location on the frame. However, for most systems, shock mounting and activation has little to do with the basic performance of the system.
The first general category is a pivotless design. With such a design, a shock is mounted between the seat stays and the seat tube, and wheel movement is achieved by simply allowing the chain stays to flex. Since the stays flex rather than rotate on a pivot, the amount of possible travel is limited. The chain stays are usually mounted low on the frame at the bottom bracket (pedal spindle) such that the wheel path is moving forward as it travels upward. Although the pivotless design may be constructed to be relatively lightweight and maintenance free, the design has limited travel and a poor wheel trajectory.
The second general category is a single pivot design. This design is similar to how many motorcycles are currently constructed. With such a design, a rear triangle or swing arm member connects the wheel to the frame. The swing arm swings on a single pivot point. The location of the pivot varies widely from one design to another and has a significant effect on how the system operates. Essentially, the location of the pivot point is a function of compromises. Locating the pivot close to the bottom bracket permits easy frame construction and minimal increase in chain length as the wheel moves through its travel. However, the wheel trajectory is poor. Raising the pivot higher on the frame improves trajectory, but the chain length problem worsens and the frame's structural integrity is compromised, especially if the pivot is also moved forward to achieve a long swing arm (the distance between the wheel and the pivot). A longer swing arm has a more constant trajectory direction, whereas a short swing arm tends to cause dramatic changes to the pro- and anti-squat effects as the wheel moves through its travel.
A variation of the single pivot design is to incorporate the bottom bracket (pedal spindle) into the swing arm. This is commonly referred to as a unified rear triangle (URT). This variation alleviates the chain length problems, but when the pivot is located forward and up on the frame, the rider's weight on the swing arm begins to interfere with the suspension characteristics. Hence, such designs typically have poor chassis roll control, which may only be reduced by compromising other features.
The third general category is a four-bar link. Such systems are described in, for example, U.S. Pat. Nos. 5,452,910 and 5,791,674, the complete disclosures of which are herein incorporated by reference. In general, a four bar link system essentially locates the wheel on one leg of a parallelogram. This provides the wheel with a more linear trajectory and may be configured to achieve rearward movement. However, the chain length problem occurs if this is taken too far. Further, mounting a shock to such systems is often a difficult task, and only adds more pivots to an already complicated design. Hence, such systems are often heavy, require significant maintenance, and experience a large degree of frame flex.
Hence, the invention relates to a new type of suspension that takes into consideration various operating parameters and riding conditions. In this way, the features of the invention are useful in mitigating bump impact while also providing chassis roll control and permitting excellent climbing abilities, ease of construction, and low maintenance, among other features.